First of all, only X11 clients can manipulate the window stacking order and ask to be placed at the bottom of the stack programmatically; this cannot be done with Wayland, where the user must be involved with any stacking operation.
Second of all, “widgets” on the desktop are not identified as a special case by GNOME, so they would just be windows. One thing that would immediately break is dynamic workspaces, as then no workspace would ever be closed if it contained “widgets”.
GNOME simply does not support desktop “widgets”. If we ever wanted to support them, they would likely be a separate, specialised workspace, and would have a special window type.